Lisa @ School Family writes: Hi there- That's difficult to answer without knowing the degree of these issues and also learning a bit about the background. Has there been an abrupt change in behavior? Hate to say it but most teens lie to some degree-- i.e. lying by omission and are defiant in varying degrees. Think the key is to have very clear expectations and consequences. Here is an article that may be helpful: Help your teenager develop responsible habits. If the behavior is a big change and extreme then it's a good idea to talk to a counselor and get professional help.